ARCHIVES OF BIOCHEMISTRY AND BIOPHYSICS Vol. 200, No. 1, March, pp. 245-252, 1980 Model Studies of the a-Peroxidase System: Formation of an Electronically Excited Product’ MARCELA HAUN, NELSON DURAN,’ OHARA AUGUSTO, AND GIUSEPPE CILENTO Department ofBiochemistry, Instituto de Qui’mica, Universidade de Sdo Paulo, C.P. 20.780, Sdo Paulo, Brazil Received June 11, 1979; revised September 14, 1979 Horseradish peroxidase-as an oxidase-converts propanaldehyde to acetaldehyde and formic acid. To some extent the enzyme also acts upon linear acids, thus mimicking even better the a-peroxidase activity of higher plants. In these reactions an electronically excited species-presumably the aldehyde-is generated, as revealed by sensitized emission. The species is long-lived; in accord with its triplet nature heavy substituents are required in the acceptor for efficient sensitization. Energy transfer occurs noncollisionally and does not appear to proceed by a long-range Forster-type T-S mechanism. A long-range triplet- triplet exciton transfer to an upper triplet state of the acceptor is proposed; then ISC occurs to the fluorescent state of the acceptor. Biological compounds which might originate from excited aldehydes are pointed out. a-Peroxidase converts a long chain fatty acid to the next lower aldehyde and CO, (1, a: R-C-C& 0 0 H2 ‘OH + 2H202 a-peroxidase ,R- C< H + CO2 + 3H20 The lower aldehyde and CO, are the prod- hydroperoxide intermediate-formally a ucts expected from the cleavage of an in- precursor of the dioxetane-has been pre- termediate dioxetane and evidence for an CY- sented (2): R-CH2 -C& 0 ‘OH Since dioxetanes are known to cleave, giv- might be generated in an excited elec- ing excited products (3, 4), the aldehyde tronic state. ’ This work was supported by FINEP (Financiadora de Pesquisas e Projetos, Rio de Janeiro), by CNPq (Conselho National de Pesquisas, Brasilia), and by FAPESP (Funda@o de Amparo a Pesquisa do Estado de Sao Paula). The authors are indebted to Dr. F. Quina for a critical reading of the manuscript. One of the authors (G.C.) thanks the John Simon Guggenheim Memorial Foundation for a Fellowship. Work being carried out in this laboratory has already shown that several reactions which yield products of the type expected from the cleavage of a dioxetane inter- mediate can be catalyzed by horseradish peroxidase (HRP)3 acting as an oxidase to * Present address: Instituto de Quimica, Univer- sidade Estadual de Campinas, Caixa Postal 1170, Campinas, SP, Brazil. 3 Abbreviations used: Me,SO, dimethylsulfoxide; teti-BuOK, potassium t-butoxide; HRP, horseradish peroxidase; DBAS, 9,10-dibromoanthracene-2- sulfonate; S, singlet; T, triplet; ISC, intersystem crossing; Z, atomic number; F, fluorescence. 245 0003-9861/80/030245-08$02.00/O Copyright 0 1980 by Academic Press, Inc. All rights of reproduction in any form reserved.